1. Field of the Invention
This invention relates to heating systems, and more particularly to a method and apparatus for controlling conventional heating apparatus and solar heating apparatus of a heating system to enable the use of solar heating whenever possible.
2. Description of the Prior Art
Various heating systems employing solar energy have been proposed as a way of conserving fuel and minimizing heating costs. However, in most areas of the country, there can be sufficiently long periods of deep cold weather with no solar input. Thus, it is recognized that a heating system utilizing 100 percent solar energy is not economically practical. On the other hand, a heating system using solar energy for 40 to 70 percent of the total annual heat requirements is feasible if such heating system has a source, other than solar, capable of supplying 100 percent of the maximum heating requirements when necessary. Thus, the combination of a conventional heating system with solar heat supplement is one probable heating method for the future.
Efficient operation of the combination of a solar heating system with a conventional heating system depends upon using solar energy whenever possible, and automatically switching to heat from the conventional heating source only when solar heat is inadequate to meet system heating requirements. This dictates a need for establishing criteria for switching from solar heat to furnace heat and from furnace heat to solar heat.
The need to switch from solar heat to furnace heat can be determined from the load requirements for the system. That is, assuming the system is employing solar heating, then if a demand for heat exists for a long duration, say 60 minutes or more, or if a demand for heat is being provided 90 percent of the time, the solar source is very nearly inadequate to satisfy the heating demand. Accordingly, a decision to switch to the conventional heating system is in order. Thus, switching from solar heating to conventional heating apparatus can be effected by monitoring the load requirements of the system.
The problem of switching from furnace heat to solar is more complex in that solar water temperature is not a sufficient changeover criterion. For example, it is possible to supply the heating requirement in mild weather with solar water at a relatively low temperature such as 115.degree. F. It is also possible that the solar water temperature could rise, during several clear days, to 150.degree. F in relatively cold weather and that this 150.degree. F water temperature would adequately supply the heat requirement. Accordingly, efficient use of solar heating requires a method for comparing heat load with solar source capability.
Therefore, it would be desirable to have a way of controlling apparatus of a combined solar and conventional heating system which enables the use of solar heating whenever possible, and which automatically switches to the conventional heating apparatus only when the solar heating source is inadequate to meet system heating requirements.